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FORK(2)			   Linux Programmer's Manual		       FORK(2)

       fork - create a child process

       #include <unistd.h>

       pid_t fork(void);

       fork()  creates	a new process by duplicating the calling process.  The
       new process, referred to as the child, is an  exact  duplicate  of  the
       calling	process,  referred  to as the parent, except for the following

       *  The child has its own unique process ID, and this PID does not match
	  the ID of any existing process group (setpgid(2)).

       *  The  child's	parent	process ID is the same as the parent's process

       *  The child does not inherit  its  parent's  memory  locks  (mlock(2),

       *  Process  resource  utilizations (getrusage(2)) and CPU time counters
	  (times(2)) are reset to zero in the child.

       *  The child's set of pending  signals  is  initially  empty  (sigpend‐

       *  The  child  does  not	 inherit semaphore adjustments from its parent

       *  The child does not inherit record locks from its parent (fcntl(2)).

       *  The child does not inherit timers  from  its	parent	(setitimer(2),
	  alarm(2), timer_create(2)).

       *  The  child  does not inherit outstanding asynchronous I/O operations
	  from its parent (aio_read(3), aio_write(3)), nor does it inherit any
	  asynchronous I/O contexts from its parent (see io_setup(2)).

       The  process  attributes	 in  the  preceding  list are all specified in
       POSIX.1-2001.  The parent and child also differ	with  respect  to  the
       following Linux-specific process attributes:

       *  The  child does not inherit directory change notifications (dnotify)
	  from its parent (see the description of F_NOTIFY in fcntl(2)).

       *  The prctl(2) PR_SET_PDEATHSIG setting is reset  so  that  the	 child
	  does not receive a signal when its parent terminates.

       *  The  default	timer slack value is set to the parent's current timer
	  slack value.	See the description of PR_SET_TIMERSLACK in prctl(2).

       *  Memory mappings that have been marked with the madvise(2) MADV_DONT‐
	  FORK flag are not inherited across a fork().

       *  The	termination  signal  of	 the  child  is	 always	 SIGCHLD  (see

       *  The port access permission bits set by ioperm(2) are	not  inherited
	  by the child; the child must turn on any bits that it requires using

       Note the following further points:

       *  The child process is created	with  a	 single	 thread—the  one  that
	  called  fork().   The	 entire virtual address space of the parent is
	  replicated in the child, including the states of mutexes,  condition
	  variables,  and other pthreads objects; the use of pthread_atfork(3)
	  may be helpful for dealing with problems that this can cause.

       *  The child inherits copies of the parent's set of open file  descrip‐
	  tors.	  Each	file  descriptor  in the child refers to the same open
	  file description (see open(2)) as the corresponding file  descriptor
	  in  the parent.  This means that the two descriptors share open file
	  status flags, current file offset, and signal-driven I/O  attributes
	  (see the description of F_SETOWN and F_SETSIG in fcntl(2)).

       *  The  child inherits copies of the parent's set of open message queue
	  descriptors (see mq_overview(7)).   Each  descriptor	in  the	 child
	  refers to the same open message queue description as the correspond‐
	  ing descriptor in the parent.	 This means that the  two  descriptors
	  share the same flags (mq_flags).

       *  The  child  inherits	copies	of  the parent's set of open directory
	  streams (see opendir(3)).  POSIX.1-2001 says that the	 corresponding
	  directory  streams  in  the parent and child may share the directory
	  stream positioning; on Linux/glibc they do not.

       On success, the PID of the child process is returned in the parent, and
       0  is returned in the child.  On failure, -1 is returned in the parent,
       no child process is created, and errno is set appropriately.

       EAGAIN fork() cannot allocate sufficient memory to  copy	 the  parent's
	      page tables and allocate a task structure for the child.

       EAGAIN It was not possible to create a new process because the caller's
	      RLIMIT_NPROC resource limit was  encountered.   To  exceed  this
	      limit,  the  process  must  have either the CAP_SYS_ADMIN or the
	      CAP_SYS_RESOURCE capability.

       ENOMEM fork()  failed  to  allocate  the	 necessary  kernel  structures
	      because memory is tight.

       ENOSYS fork()  is not supported on this platform (for example, hardware
	      without a Memory-Management Unit).

       SVr4, 4.3BSD, POSIX.1-2001.

       Under Linux, fork() is implemented using copy-on-write  pages,  so  the
       only  penalty  that it incurs is the time and memory required to dupli‐
       cate the parent's page tables, and to create a  unique  task  structure
       for the child.

       Since  version  2.3.3,  rather than invoking the kernel's fork() system
       call, the glibc fork() wrapper that is provided as  part	 of  the  NPTL
       threading  implementation  invokes clone(2) with flags that provide the
       same effect as the traditional system  call.   (A  call	to  fork()  is
       equivalent  to  a  call	to clone(2) specifying flags as just SIGCHLD.)
       The glibc wrapper invokes any fork handlers that have been  established
       using pthread_atfork(3).

       See pipe(2) and wait(2).

       clone(2),   execve(2),  exit(2),	 setrlimit(2),	unshare(2),  vfork(2),
       wait(2), daemon(3), capabilities(7), credentials(7)

       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2013-03-12			       FORK(2)

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